replace_words.rb

def replace_words(dict, sentence)
  regex = /\A(#{dict.join('|')})/

  sentence.split.inject([]) do |new_words, word|
    if match_data = word.match(regex)
      new_words << match_data.captures[0]
    else
      new_words << word
    end
  end.join(' ')
end

puts replace_words(['cat', 'bat', 'rat'], "the cattle was rattled by the battery")

# -------------------------------------
# James solution
dict = ["cat", "bat", "rat"]
sentence = "the cattle was rattled by the battery"

def replace_words2(dict, sentence)
  tokens = sentence.split(' ').map do |word|
    prefix(dict, word)
  end
  puts tokens.join " "
end

def prefix(dict, word)
  dict.each do |prefix|
    return prefix if word.match(/^#{prefix}/)
  end
  word
end

x = replace_words2(dict, sentence)
puts "RESULT: #{x}"